Ventricular assist devices are blood pumps, which assist blood circulation when a subject's heart is incapable of providing adequate blood circulation. FIG. 1 illustrates a conventional ventricular assist device (VAD) with extra-corporeal power and control components. The VAD 2 is implanted in a subject's body 4 near the heart 3. The VAD 2 is coupled by a percutaneous cable 6 to extra-corporeal components including a controller 8 and a power source. The power source may include and alternating current (AC) power source (not shown) and a pair of rechargeable batteries 10, 12, for example.
Power cables 14, 16 couple the power sources to the controller 8. Conventionally, one end of each power cable 14, 16 includes a connector coupled to the power source and another end of each power cable 14, 16 includes another connector coupled to the controller 8. Normally, more than one rechargeable battery 10, 12 are coupled to a VAD controller 8, as shown, to provide improved safety. During operation of the VAD 2, when the controller 8 that is supplying power to the VAD 2 is disconnected from an AC power cable (not shown), two batteries 10, 12 remain coupled to the controller 8 to ensure that sufficient and continuous power is provided to the VAD 2. When one of the rechargeable batteries 10, 12 (e.g., battery 10) is disconnected from the controller 8 to be charged, the other battery (e.g., battery 12) remains connected to the controller 8 and provide uninterrupted power to the controller 8 and blood pump 2. Within a short time after one battery 10 is disconnected from the controller 8, a fully charged standby battery is usually connected to the controller 8 via connecting cables 14. The system then resumes operation with at least two power sources connected to the controller.
Normally, the two batteries 10, 12 are substantially similar or identical with the same capacity, size and weight. This type of conventional system generally draws power from one battery (e.g. 10) at a time until it is discharged to certain level. Then the system urges the patient to replace the substantially discharged battery with a fully-charged standby battery. During this period, the second battery (e.g. 12), which is connected to the controller, stands idle and remains at full capacity. However, in the conventional system, the time interval between changing batteries is usually determined by the life time of a single battery, rather than the sum of the life time of the two batteries. For example, if one battery 10 lasts five hours, then the patient is typically instructed to change battery every five hours, even though the second battery 12 is still capable of maintaining the system in normal operation for the next five hours. As a result, a patient using the conventional system generally carries a redundant battery of the same size and weight as an operational battery all the time. This causes inconvenience and burden that detrimentally affects the patients' normal activities.
The conventional configuration as shown in FIG. 1, includes at least three extra-corporeal components including the controller 8, a first rechargeable battery 14 coupled to the controller 8 and a second rechargeable battery 16 separately coupled to the controller 8. These extra-corporeal components, which are carried by the subject, severely limit the subject's mobility and comfort. Moreover, the power cables 14, 16 that are coupled between controller 8 and batteries 10, 12 are prone to mechanical wear and include connectors that may eventually fail after being repeatedly connected and disconnected to the controller 8 and batteries 10, 12, for example, many times in a day every day. It is also possible that a connector of a power cable 14, 16 may loosen or that the power cable may become completely disconnected from the battery 10, 12 or controller 8. These failure modes impose a potentially fatal risk of power interruptions and/or control signal interruptions to a VAD that could prevent the VAD from operating.